This application relates generally to the cooling of motors for vapor compression systems incorporated in air conditioning and refrigeration applications. More specifically, this application relates to cooling semi-hermetic motors for vapor compression systems.
Vapor compression systems can use more compact motors operating at higher rotational speeds to provide power to components. By using more compact motors, a reduction in the size of the systems can be obtained. However, some challenges associated with operating motors at higher rotational speeds include the generation of friction between the motor shaft and bearings and windage losses. Windage is a frictional force created between the rotating rotor of the motor and the environment surrounding the rotor, typically air or a working media, such as refrigerant vapor in the case of a hermetic driveline. Windage can create heat and reduce the operational efficiency of the motor. Therefore, effective cooling of these motors is highly desirable.
Cooling of a motor stator may be achieved by use of a cooling coil surrounding the stator, the coil receiving liquid refrigerant from a condenser of a vapor compression system. The coil is typically integrated in the stator housing. Due to contact with the warm surfaces of the stator and its housing, the refrigerant evaporates in this coil and cools the stator. An example is disclosed in U.S. Pat. No. 6,070,421. In addition, a similar refrigerant circuit can also be used to cool electronic components used for the variable speed drive (VSD), bearing electronics, when such components are disposed on the motor housing that can be a “cold plate” for these components.
Motor components that are not in sufficiently close proximity with the motor housing (motor windings, bearings, etc.) require other cooling arrangements. As in traditional semi-hermetic motors, a known approach is to sweep or direct cold vapor or gas through the motor cavity. However, particular arrangements of components must be provided to supply and circulate the cold gas in the motor. In one traditional semi-hermetic motor, part or all of the gas provided to compressor suction is provided to pass over or through the motor cavity prior to reaching compressor suction.
A further cooling arrangement is disclosed in U.S. Pat. No. 7,181,928, in which some cold gas is taken from the evaporator and drawn into the compressor suction. The pressure difference required to move the gas through the motor cavity is provided by the venturi effect that is produced at the inlet of the impeller of a centrifugal compressor.
In a further arrangement, cold gas evaporated in a coil surrounding the stator is used to cool the motor cavity. In this arrangement, a control device is used with respect to the supply of liquid refrigerant to the coil, so that all of the liquid is evaporated at the coil outlet. This control device can be a thermal expansion valve similar to those used in conjunction with “Dry-expansion” evaporators, or a more or less equivalent system (e.g., a combination of solenoid valves controlled by a temperature sensor, etc.) to avoid sending liquid into the motor.
U.S. Pat. No. 6,070,421 discloses a two stage system with an intercooler, in which the flash gas from the intercooler is used to sweep or to be directed through the motor housing. In addition, the gas evaporated in the coil surrounding the stator that is also directed through the housing is then vented at the inter stage pressure. As disclosed in the previous arrangement, an expansion valve is provided to ensure all of the liquid refrigerant is evaporated from the coil, as any remaining liquid could damage motor components.
While the systems as described provide viable results, the systems also have drawbacks.
For example, use of an expansion device at the inlet of a cooling coil to ensure all of the liquid refrigerant is evaporated from the coil also ensures pressure in the motor cavity self-adjusts to a level slightly above suction pressure or inter stage pressure, depending upon the application. The self-adjustment provides gas to be effectively directed through the cavity of the motor housing to cool the cavity. However, the system is not thermally optimized: complete evaporation of the refrigerant provides a reduction in heat transfer in the coil as compared to refrigerant at the coil outlet that is in a two phase state. Also, the gas refrigerant sent into the motor tends to be somewhat superheated, resulting in less efficient cooling in the motor cavity. In addition, in the system providing gas refrigerant at a level slightly above inter stage pressure, evaporation occurs at a higher temperature, which reduces the amount of cooling that can be provided. Operating the motor in gas at an increased internal pressure level also increases the amount of friction (and heat) generated by the gas refrigerant, undermining the initial purpose of cooling the motor.
U.S. Pat. No. 7,181,928 does not include a thermostatic expansion valve at the inlet of the stator cooling coil, containing only a fixed orifice sized such that the amount of liquid refrigerant directed into the cooling coil surrounding the stator is substantially larger than the amount that needs to be evaporated to reject the stator heat. This arrangement results in two phase flow at the coil outlet. Two phase flow of refrigerant improves the heat transfer in the coil, providing better cooling to the stator; but a consequence is that the two phase refrigerant flowing out of the coil cannot be sent directly into the motor. Introducing liquid refrigerant into a high speed motor presents the risk of damaging some components of the motor, e.g., by erosion generated by liquid droplets. In response to the risk of damage, the '928 patent discloses the two phase refrigerant exiting the coil is first sent back to the evaporator to separate the liquid from the gas; then some cold gas separated by the evaporator is returned to the motor cavity.
Additionally, while the '928 patent is well suited and proven for compressors without Pre-Rotation Vanes (PRV), or using a PRV for capacity reduction, an alternative to the PRV is to use a Variable Gap Diffuser (VGD) as a capacity reduction device. When a VGD is used for capacity reduction, the reduction of pressure at compressor suction at a partial load is not large enough to draw a satisfactory amount of gas refrigerant through the motor cavity, resulting in insufficient motor cooling.
Therefore, what is needed is a cooling arrangement allowing each of the following advantages to occur simultaneously:                Accommodate a sufficiently large supply of liquid refrigerant to the coil surrounding the stator, to optimize the stator cooling by virtue of two phase flow out of the coil.        Provide easy and efficient sweep or directed flow of cold gas or cooling vapor through the motor cavity.        Prevent introduction of liquid refrigerant into the motor cavity.        Provide the possibility of venting of the vapor or gas refrigerant from the motor housing at or close to suction pressure to maintain reduced temperature vapor or gas directed through the motor cavity, as well as maintaining reduced vapor or gas friction losses.        